In the last two decades, a great deal of work has gone into the development of solid state damage track detectors (SSTD) [R. L. Fleischer, P. B. Price and R. M. Walker, Annual Rev. Nucl. Sci., 15, p. 1, 1965] for applications in various areas of radiation physics. Perhaps in no other area was the impact of these new SSTD's more striking that in neutron dosimetry where there was a shortage of other promising techniques which could be developed. About ten years ago, neutron detectors using "fission radiators" against SSTD's, such as polycarbonate ones, received a great deal of attention. The fission radiators were necessary because these SSTD's were insensitive to recoil protons but could easily detect fission fragments emitted by the radiators when exposed to neutrons.
From the viewpoint of neutron dosimetry, especially personal neutron dosimetry, there were two main problems with the fission foil SSTD's. One was the need to use fissionable materials. Aside from the regulatory problems regarding the handling of fissionable materials, the radiators emitted gamma radiation which provided unnecessary exposure to the user of the dosimeter--even though this was usually small. The second was the lack of a fissionable material which could respond primarily to neutrons in the energy region between say 10 keV to 0.5 MeV. Nature has simply not provided us with materials having the desired neutron cross-section. Unfortunately, this energy region is quite important for neutron dosimetry because a large fraction of the neutron dose in radiation protection can be delivered by such neutrons.
The advent of more sensitive plastics such as the amphorous thermoset plastic called CR-39 [B. G. Cartwright, E. K. Shirk and P. B. Price, Nucl. Inst. Meth., 153, p. 457, 1978] eliminated the problem of fission radiators. Especially when used with electrochemical etching [L. Tommasino, "Electrochemical Etching of Damaged Track Detectors by H.V. Pulse and Senosoidal Waveforms", Report No. RT/PROT(71), 1970, Comitato Nazionals Energia Nucleare, Rome], these plastics can detect recoil carbon ions and protons from interactions with neutrons as low in energy as 100 keV [R. V. Griffiths, J. H. Thorngate, K. J. Davidson, D. W. Rueppel, J. C. Fisher, L. Tommasino, G. Zapparolli, Rad. Prot. Dos., Volume 1, page 61, 1981]. However, it is clear there is a lower energy limit below which SSTD's are insensitive. Below of the order of about 100 keV, neutrons would probably not have enough energy to damage the detector medium to the extent that chemical reagents can identify the site and preferentially attack the damage. It appears that to measure such neutrons, the detector cannot be passive, but must contain stored energy in some form. In such a detector, the stored energy can be utilized to amplify the effect of the secondary particles from the neutron interactions in much the same way that a Geiger counter uses electrical energy to amplify the initial ionizations or a cloud chamber uses stored energy in the form of supercooled vapor to visualize particle tracks.
A method of providing stored energy, in the case of a liquid medium, is to superheat the liquid. One of the earliest applications of using superheated liquids for the detection of radiation was the bubble chamber, invented by Glaser [D. A. Glaser, Phys. Rev. 87, p. 665, 1952]. In this detector, ionizing particles, traversing a superheated liquid, cause bubbles to grow along the track to a size at which they may be photographed before uncontrolled boiling occurs throughout the medium. The liquid was superheated by dropping suddenly the pressure inside the containment using a piston or a diaphragm. Other methods of superheating liquids were used in work connected with so-called "cavitation studies". Researchers such as Lieberman [D. Lieberman, "Radiation-Induced Cavitation", Phys. of Fluids 2, p. 466, 1958], Hahn [B. Hahn and R. N. Peacock, "Ultrasonic Cavitation Induced by Neutrons", Nuova Cemento XXVIII, p. 1880, 1963], West [C. West and R. Howlett, "Some Experiments on Ultrasonic Cavitation using a Pulsed Neutron Source", Brit. J. Appl. Phys., 1, p. 247, 1968], and Skripov [V. P. Skripov, "Metastable Liquids", Halsted Press, John Wiley and Sons, New York, 1974], were experimenting with the effect of radiation on liquids under stress. They were superheating the liquid, such as acetone or Freon (R) compounds, or allowing drops on one liquid to rise in another liquid having a temperature gradient until the drops were superheated. They found that radiation such as gamma rays or neutrons will trigger the volatilization of the superheated liquid. These events produced audible sound which could be detected by microphones.
R. E. Apfel ["The Superheated Drop Detectors", Nucl. Inst. Meth. 162, p 603, 1979; U.S. Pat. No. 4,143,274, Mar. 6, 1979; "Photon-Insensitive, Thermal to Fast Neutron Detector", Nucl. Inst. Meth. 179, p. 615, 1981; "Thermal to Fast Neutron Detector with Superheated Drops", Eighth DOE Workshop on Personnel Neutron Dosimetry, Pacific Northwest Laboratory Report PNL-SA-9950, p. 36, 1981, and U.S. Pat. No. 4,350,607, Sept. 21, 1982] proposed a method of exploiting the phenomenon of radiation induced cavitation for application in neutron dosimetry. His approach was to place the superheated liquid drops into a viscous fluid or a soft gel in order to immobilize the droplets for a sufficiently long period of time to be usable as a neutron dosimeter. When neutrons strike the liquid drops, the drops volatilize and produce gas bubbles which rise through the medium to collect above it or which expand the entire medium by an equivalent volume. Apfel proposed using the volume of evolved gas, as a measure of the neutron dose. Apfel's studies have led to a better understanding of the dynamics of the process of vapour bubble nucleation, [R. E. Apfel, B-T. Chu and J. Mengel, "Superheated Drop Nucleation for Neutron Detection", Applied Scientific Research 38, p. 117, 1982; S. C. Roy and R. E. Apfel, "Semi-Empirical Formula for the Stopping Power of Ions", Nuclear Instruments and Methods in Physics Research B4, p. 20, 1984] and he has developed neutron detectors using superheated drops in soft gels [R. E. Apfel and S. C. Roy, "Instrument to detect vapour nucleation of superheated drops", Review of Scientific Instruments 54, p. 1387, 1984; R. E. Apfel and S. C. Roy, "Investigations on the Applicability of Superheated Drop Detectors in Neutron Dosimetry", Nuclear Instruments and Methods in Physics Research 219, p. 582, 1984]. Instead of using volume change as initially proposed, his detectors use a piezoelectric sensor to convert the acoustic signal emitted by the droplet explosion into an electrical signal which must be processed electronically before being counted. Thus, the operation of his detectors requires elaborate electronic instrumentation. Furthermore, this method of detection provides little spatial information about neutrons or their reaction products.